A bit patterned medium is attracting attention for its potential of increasing the recording density and capacity of a magnetic recording apparatus. The bit patterned medium has on its surface a large number of magnetic dots formed by utilizing fine patterning technology. The magnetic dots are magnetically separated from one another. Each magnetic dot is used to store one-bit of information. In the bit patterned medium, used often as a magnetic recording layer is a so-called magnetic continuous film, which is a layer of magnetic crystal grains arranged tightly.
Meanwhile, in the case where no patterning is performed on a magnetic continuous film, i.e., in the case of an as-grown film, or in the case where this magnetic continuous film is patterned into relatively large portions, the nucleation field (Hn) for magnetization reversal and the coercive force (Hc) are as small as about a few hundred Oe. However, in the case where the magnetic continuous film is patterned into fine portions, the shapes thereof so affect the magnetic characteristics to increase Hn and Hc. That is, in the case of using the magnetic continuous film, the magnetic characteristics greatly depend on the shape thereof.
A bit patterned medium has patterns on a servo region storing head position control information, etc. and a pattern on a data region in which information is to be recorded. Specifically, these regions include microstructures made of a magnetic material. The sizes of the microstructures in the servo region differ from the sizes of the microstructures in the data region. Therefore, when using a magnetic recording layer in which magnetic crystal grains are arranged with almost no gap therebetween, the regions have different magnetic characteristics such as nucleation fields (Hn) for magnetization reversal, coercive forces (He), and saturation magnetic fields (Hs).
To be more specific, since the servo region includes microstructures having greater sizes than those of the data region, the servo region has smaller Hn and Hc. This facilitates generating reverse magnetic domains due to, e.g., the stray magnetic field or thermal fluctuation. If reverse magnetic domains are formed in the servo region, it becomes difficult for a head to access a target magnetic dot.
On the other hand, the data region includes microstructures having smaller sizes. So the variations in shape, composition, crystal grain boundary, etc. have a great influence on the variation in magnetic characteristics, e.g., switching field distribution (SFD). If the SFD is made wider, a margin of a write operation of the head to write information in the magnetic dot sharply decreases, which makes it difficult to write information in a target dot alone.
It is also important to secure the single-domain characteristics of a magnetic dot in the data region. A magnetic dot that readily forms a plurality of magnetic domains in the dot easily causes a write error when the medium is incorporated into a magnetic recording apparatus.
A CoCrPt-based granular film used in a conventional perpendicular magnetic recording medium can also be used in a patterned medium. In this case, generation of reverse magnetic domains can be prevented in the servo region because the magnetic coupling between the crystal grains is weak. However, in this case, the variations in magnetic characteristics of dots increase and the single-domain characteristics cannot be secured because the magnetic coupling between the magnetic crystal grains is weak. Accordingly, unlike the conventional perpendicular magnetic recording medium, it is unfavorable to manufacture a patterned medium using the CoCrPt-based granular film.
From the foregoing, demands have arisen for a bit patterned medium capable of preventing the formation of reverse magnetic domains in the servo region, and capable of reducing the SFD and securing the single-domain characteristics in the data region.